Digital circuitry has historically been powered at a 5 VDC level. Relatively recently, however, an alternative 3.3 VDC level has been adopted to power high density, high speed integrated circuits. As a result, two different digital systems generally exist, namely, a 5 volt system that only provides a 5 VDC supply voltage and a 3.3 volt system that provides both 3.3 VDC and 5 VDC supply voltages.
Each integrated circuit can therefore be designed in two different versions, one version of which would operate in a 5 volt system and another version of which would operate in a 3.3 volt system. As will be apparent, the manufacture of two versions of the same integrated circuit is disadvantageous since, among other things, the two versions would decrease the economies of scale typically associated with the mass production of an integrated circuit. Therefore, universal integrated circuits capable of operating in either a 3.3 volt system or a 5 volt system have been designed.
By way of example, personal computers, workstations, servers and the like commonly include a peripheral component interconnect (PCI) bus to provide a high-speed data path between the central processing unit and a number of peripheral devices connected to the PCI bus. These peripheral devices can include a display, a disk drive, a network controller and the like. In order to communicate via the PCI bus, a computer card, such as an adapter card and, more particularly, a modem card, a network controller card, a video card, a sound card or the like, is plugged into an expansion or adapter slot defined by the PCI bus. Upon being plugged into an expansion slot, the adapter card is connected, typically by means of an edge connector, with the address and data lines of the PCI bus. In addition, the adapter card is electrically connected to one or more power rails provided by the PCI bus that supply power to the components mounted upon the adapter card and to the associated peripheral devices. A PCI bus commonly includes a 5 volt power rail. In a 3.3 volt system, however, the PCI bus will include both a 5 volt power rail and a 3.3 volt power rail.
In instances in which the PCI bus has both 5 volt and 3.3 volt power rails, the adapter card can be provided with both 5 VDC and 3.3 VDC in a direct manner via the edge connector. Alternatively, the adapter card can include a regulator to convert a portion of the 5 volt supply voltage to 3.3 volts. As such, of the components of the adapter card that operate at a 3.3 VDC power level, a first predefined subset of the components are powered by the 3.3 volt supply voltage provided by the regulator, while a second predefined subset of the components are directly powered by the 3.3 volt power rail. While this adapter card can be readily plugged into a PCI bus having both 5 volt and 3.3 volt power rails, this adapter card is not universal since the adapter card cannot properly function if the adapter card is plugged into a PCI bus that only provides a 5 volt power rail. In this regard, while the regulator would continue to power the first predefined subset of components with 3.3 volts, the second predefined subset of components, that are designed to operate at a 3.3 volt power level and are otherwise directly powered by the 3.3 volt power rail, will go unpowered.
As will be apparent, it would be most advantageous for computer logic circuitry, such as the components mounted upon an adapter card, to be universal in that the computer logic circuit can operate in both a 5 volt system and a 3.3 volt system. In addition, it would be advantageous for computer logic circuitry to draw power from both the 5 volt power rail and the 3.3 volt power rail in instances in which both power rails are available in order to more evenly distribute the power loading.
In this regard, each power supply of a computer system can provide a predetermined maximum amount of power to the various adapter cards, peripheral devices and the like that are connected to a respective power rail. For example, a typical power supply of a computer system can provide up to 200 to 250 watts of power at 5 volts and 200 to 250 watts of power at 3.3 volts. While this maximum amount is generally sufficient, it may be exhausted, especially upon powering up, if the loads are not judiciously balanced. By way of example, in an attempt to be universal, an adapter card may be exclusively powered from the 5 volt power rail, even in a 3.3 volt system. For example, the components onboard the adapter card that operate at a 5 volt power level can be powered directly from the 5 volt power rail, while the components onboard the adapter card that operate at a 3.3 volt power level can be powered by a regulator that converts the 5 volt supply voltage to 3.3 VDC. While each adapter card is generally designed to draw no more than a predetermined maximum amount of power, such as 25 watts, the maximum power provided by the 5 volt power supply will be exhausted more quickly by powering all of the components of the adapter card from the 5 volt power rail than if the adapter cards were powered from both the 5 volt power supply and the 3.3 volt power supply. Accordingly, a system having a substantial number of adapter cards may be unable to be properly powered up since the collective power drawn from the 5 volt power rail by the plurality of adapter cards of this example may exceed the power rating of the 5 volt power supply.